Explicitly bidirectional pattern synonyms for 7.8.3?

[Dr. ERDI Gergo]

Hi,

I've spent the last couple evenings implementing explicitly-bidirectional 
pattern synonyms, partly while waiting for someone to help out with #9023. 
Explicitly-bidirectional pattern synonyms are described in #8581.

My question is: now that I have this mostly working, is this something 
that could be added to 7.8.3, or is this too much of a new feature for a 
point release? I'd like to know because then I can prioritize finishing 
the work (it still needs some tests added and also someone to review the 
changes).

Thanks,
 	Gergo